This invention relates generally to pulp processing and, more particularly, to the final bleaching and washing stages of a multiple stage pulp processing system.
In the pulping industry, environmental and economic concerns dictate that water be conserved and effluent volumes be reduced. Reductions in the amount of fresh water consumed, as well as in effluent volumes may have additional beneficial consequences in the form of reductions in amounts of fibers and chemicals used, reductions in heat losses, and overall reductions in operating costs. To achieve these benefits, however, it is critical that these matters be handled correctly and efficiently.
One area of conventional pulp processing which utilizes considerable amounts of fresh water is in the washing operations following the last two pulp bleaching stages. Another area where large amounts of fresh water are used is in pulp drying apparatus.
In the prior art, one current practice is to conserve water by extensive recycling in the pulp dryer, and by using excess white water effluent discharge from the dryer to wash pulp in the washer following the last bleaching stage. In this manner, effluent from the pulp dryer can be substantially reduced. However, fresh water is still required in the next-to-last washer accompanying the next-to-last bleaching stage. The prior art arrangement described above causes corrosion problems in the dryer due to the build up of salt, pitch and resin concentrations which will eventually effect pulp quality. In addition, this configuration can result in build up of other potentially harmful substances that could cause injury to intermediate or final users of the product.
It has now been discovered that the problem of build up of various harmful deposits in the white water system of the pulp dryer (or other pulp processing apparatus such as a paper making mill), may be effectively dealt with by using the white water effluent discharge as the wash water in the second-to-last washer stage, and by using fresh water in the last washer stage. In this way, most of the harmful white water compounds will be washed out in the last washer. As a result, it has been found that fresh water consumption and effluent volumes may be maintained at a lower level, and contamination of the white water discharge as well as the pulp leaving the dryer or other processing apparatus may be reduced up to about 40% of the contamination level currently experienced in the prior art systems.
Another advantage of the arrangement described herein is a reduction in corrosion within the pulp dryer which, normally, is not made of the higher grade alloys used in the construction of, for example, the pulp bleaching apparatus.
While there may be other ways to reduce the concentrations of various harmful compounds in the white water system, the only one that has been found to have reduced water consumption and effluent volume is one which requires the addition of still another washer stage between the last bleaching stage and the pulp dryer or other pulp processing equipment. However, the cost of this measure is substantially higher, both from an investment standpoint as well as an operational standpoint, particularly since this additional washer would have to be made of high grade steels to prevent excess corrosion.
Additional details and objects of the invention will become apparent from the detailed description of the invention and claims which follow.